Compositions and methods for making microporous ceramic materials

ABSTRACT

The present invention provides methods for making a microporous ceramic material using a metal silicon powder and including a reaction sintering process of the silicon. A material for forming a microporous ceramic material used in these methods includes a metal silicon powder, a silicon nitride powder and/or a silicon carbide powder, and if desired, a yttrium oxide powder and/or an aluminum oxide powder. These methods can make a microporous ceramic material that can be used preferably as a gas or liquid filter, a catalyst carrier or a support of a gas separation membrane.

[0001] This application claims priority to Japanese Patent ApplicationsNo. 2002-248458, filed Aug. 28, 2002, and No. 2002-286830, filed Sep.30, 2002, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to compositions and methods formaking microporous materials made of nonoxide ceramics.

[0004] 2. Description of the Related Art

[0005] Various microporous ceramic materials are used as a filter, acatalyst carrier, a substrate of a separation membrane used as a gasseparating member or the like.

[0006] In recent years, nonoxide ceramics having silicon such as siliconnitride and silicon carbide as the main constituent gain attention as amicroporous ceramic material used for these applications. For example,the microporous ceramic materials having silicon nitride as the mainconstituent have excellent heat resistance and thermal shock resistance,and are suitable for use under a high temperature environment (300° C.or more, for example, 600° C. or more and less than 1000° C.).

[0007] Japanese Laid-Open Patent Publication No. 8-133857 describes amicroporous ceramic material comprising silicon nitride which is usedunder a high temperature environment as a gas or liquid filter or acatalyst carrier and a method for making the same.

[0008] Conventionally, the microporous nonoxide ceramic materials usedfor the above-listed applications are made in the following manner (seethe above-mentioned Laid-Open Publication): a powder of the nonoxideceramic (e.g., silicon nitride powder) is used as the raw material, andthe powder was molded into a predetermined shape and sintered. However,the nonoxide ceramic powder such as silicon nitride is expensive,compared with common oxide ceramic powders (e.g., silica powder, aluminapowder). For this reason, the nonoxide ceramic materials such as siliconnitride obtained by sintering such a nonoxide ceramic powder are moreexpensive than oxide ceramic materials.

[0009] It is an object of the present invention to provide a microporousceramic material made of nonoxide ceramics having silicon as the mainconstituent element in a lower cost of the raw material and moreinexpensively than conventional microporous ceramic materials, inparticular, to provide a microporous ceramic material suitable for useas a filter, a catalyst carrier, a support of a separation membrane (gasseparation membrane or the like).

SUMMARY OF THE INVENTION

[0010] The present invention provides materials (compositions) andmethods for making microporous ceramic materials that are rich inmicropores and composed mainly of nonoxide ceramic at a lower cost.

[0011] One composition provided by the present invention is acomposition for forming a microporous ceramic material based on siliconnitride and/or silicon carbide. This composition includes a metalsilicon powder, in addition to at least one nonoxide ceramic powderselected from the group consisting of a silicon nitride powder and asilicon carbide powder as the raw materials constituting the ceramicmatrix. Preferably, the mixing ratio of the metal silicon powder and thenonoxide ceramic powder is 5 parts or more and less than 60 parts of themetal silicon powder with respect to 100 parts of the nonoxide ceramicpowder in the mass ratio. More preferably, the mixing ratio of the metalsilicon powder and the nonoxide ceramic powder is 25 parts or more andless than 45 parts of the metal silicon powder with respect to 100 partsof the nonoxide ceramic powder in the mass ratio.

[0012] Some of the compositions disclosed herein can contain at leastone oxide powder selected from the group consisting of a yttrium oxidepowder and an aluminum oxide powder. In this case, the content ratio ofthe metal silicon can be higher. For example, the mixing ratio of themetal silicon powder and the nonoxide ceramic powder is 10 parts or moreand less than 100 parts (more preferably, 20 parts or more and less than90 parts) of the metal silicon powder with respect to 100 parts of thenonoxide ceramic powder in the mass ratio. The content of the oxidepowder (the yttrium oxide powder and/or the aluminum oxide powder) addedto the composition containing the metal silicon powder and the nonoxideceramic powder in such a mass ratio is preferably an amountcorresponding to 2 mass % or more and less than 250 mass % of thecontent of the metal silicon powder and not more than 20 mass % of thetotal amount of the metal silicon powder, the nonoxide ceramic powderand the oxide powder.

[0013] The material (composition) for forming a microporous ceramicmaterial disclosed herein contains a metal silicon powder, in additionto a silicon nitride powder and/or a silicon carbide powder. In general,the metal silicon powder is less expensive than the nonoxide ceramicpowder. Therefore, if a microporous ceramic material is made of thematerial (composition) disclosed herein, the production cost can bereduced by the extent that the metal silicon powder is used, comparedwith the case when a microporous ceramic material is made only with thenonoxide ceramic powder as described above.

[0014] The present invention provides a method for making a microporousceramic material based on a nonoxide ceramic, using a material that cancontribute to a reduction of the production cost. More specifically,this method includes preparing any one of the compositions disclosedherein, molding the composition (material for forming a microporousceramic material) into a molded product having a predetermined shape,and subjecting the obtained molded product to reaction sintering in anatmosphere that allows nitriding. This reaction sintering can producesilicon nitride having high heat resistance from the metal siliconpowder in the molded product.

[0015] In the production method disclosed herein, a material(composition) containing the metal silicon powder that is less expensivethan the silicon nitride powder and the silicon carbide powder is used,so that the production cost is reduced, and a microporous ceramicmaterial based on a nonoxide ceramic can be made more inexpensively.Furthermore, the powder material containing the metal silicon powder canbe molded more easily than the powder material made only of the siliconnitride powder or the silicon carbide powder. Therefore, by using aconventional extrusion molding technique or the like, a microporousceramic material (molded product before firing) having a desired shapecan be easily obtained. For example, in the molding process, it ispreferable that the composition (molded product) is molded underpressure at a molding pressure set in the range from 30 MPa or more andless than 200 MPa.

[0016] Japanese Patent Publication Nos. 52-19207 and 61-38149, andJapanese Laid-Open Patent Publication Nos. 7-81909 and 11-79849 describea method for making silicon nitride by reaction sintering. However, themethods described in these publications are directed to a method formaking an ingot or a silicon nitride structure having a dense structure,and is different from the method of the present invention and is notsuitable for making a microporous ceramic material. In other words, inthe ceramic material obtained by the conventional nitriding as describedin these publications, the micropore size is in the sub-micron level orsmaller than that (typically, the average micropore size is 0.1 μm orless), and is not suitable for filters or the like. On the other hand,if the material (composition) for forming a ceramic material disclosedherein is used, a microporous ceramic material having a comparativelynarrow micropore size distribution and being rich in micropores having amicropore size of about 1 μm, typically an average micropore size or apeak value of the micropore size distribution of 0.6 μm or more and lessthan 1.6 μm, particularly preferably 0.8 μm or more and less than 1.2μm, that can be used preferably as (1) a gas or liquid filter, (2) acatalyst carrier or (3) a substrate (support) of a separation membraneused as a gas separating member or the like, can be made.

[0017] It is preferable to use a material (composition) for forming aceramic material in which the average particle sizes of both the metalsilicon powder and the nonoxide ceramic powder are in the range from 1μm or more and less than 50 μm. By using a material containing the metalsilicon powder and the nonoxide ceramic powder having such a particlesize, a microporous ceramic material that is rich in micropores having amicropore size of about 1 μm and is suitable for the above applicationscan be made easily.

[0018] Preferably, the average particle size of the metal silicon powderand the average particle size of the silicon nitride powder and thesilicon carbide powder are 1 μm or more and less than 50 μm. If amaterial to be molded containing powders having such an average particlesize is used, a microporous ceramic material that is rich in microporeshaving a micropore size of about 1 μm and is suitable for the aboveapplications can be made easily. When the yttrium oxide powder and/orthe aluminum oxide powder are contained, it is preferable that theaverage particle size of these powders is 0.1 μm or more and less than 1μm. A material containing a dispersion medium that disperses the metalsilicon powder and the nonoxide ceramic powder (the oxide ceramic powdercan be contained) is easy to use and is preferable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is an electron microscope (SEM) photograph showing thesurface structure of a microporous ceramic material of one example.

[0020]FIG. 2 is an electron microscope (SEM) photograph showing thesurface structure of a microporous ceramic material of one example.

[0021]FIG. 3 is an electron microscope (SEM) photograph showing thesurface structure of a conventional microporous ceramic material.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The technical matters, other than the contents specificallyreferred to in this specification that are necessary in order topractice the present invention can be considered as a matter of designchoice to those skilled in the art based on conventional techniques. Thepresent invention can be practiced based on the technical contentsdisclosed by this specification, the drawings and by technical commonknowledge of the art.

[0023] A material for forming a ceramic material (i.e., a compositiondisclosed herein) used in a method disclosed herein contains a siliconnitride powder and/or a silicon carbide powder (typically, only one ofeither a silicon nitride powder or a silicon carbide powder) as anonoxide ceramic component, and a metal silicon powder.

[0024] Nonoxide ceramic powders (silicon nitride powder and/or siliconcarbide powder) having an average particle size (based on microscopemeasurement or a sedimentation method) of 0.1 μm or more and less than100 μm are suitably used, and preferably 1 μm or more and less than 50μm, more preferably 1 μm or more and less than 20 μm, and mostpreferably 2 μm or more and less than 10 μm are used. When the averageparticle size is larger than 100 μm, the micropore size and the porosityof a ceramic material that has been made are too large, so that it isnot suitable for the applications shown in the above items (1) to (3).On the other hand, an average particle size of smaller than 0.1 μm isnot preferable because the ceramic material that has been made has adense structure having a small micropore size and porosity. Impuritiesmay be present in the silicon nitride powder and/or the silicon carbidepowder to be used, but it is preferable that the purity thereof is high(e.g., the content ratio of SiC or Si₃N₄ is 99 mass % or more).

[0025] A silicon nitride powder and/or a silicon carbide powder eitherof α crystal structure, β crystal structure or in an amorphous state canbe used. However, it is particularly preferable to use a β siliconnitride powder and/or silicon carbide powder, which are thermallystable. A ceramic material having a large number of micropores having asuitable size as a gas or liquid filter, a catalyst carrier or amicroporous substrate on which a ceramic separation membrane is formedon its surface can be easily made by using a nonoxide ceramic powderconstituted only by β-type powder or having a high ratio of β-type(e.g., the β-type is 50 mass % or more of the total amount of thenonoxide ceramic powder to be used.)

[0026] On the other hand, there is no particular limitation regardingthe metal silicon powder used for preparing the material (composition),as long as it has been conventionally used to make a silicon nitride byreaction sintering. For example, a metal silicon powder having aspecific surface area of 0.1 m²/g or more and less than 5 m²/g can beused preferably. The average particle size (based on microscopemeasurement or a sedimentation method) is 1 μm or more and less than 50μm, more preferably 1 μm or more and less than 20 μm, and mostpreferably 2 μm or more and less than 10 μm.

[0027] Impurities may be present in the metal silicon powder, but it ispreferable that the purity of the metal silicon powder to be used ishigh. For example, a metal silicon having a Si content ratio of 95 mass% or more (i.e., the content ratio of impurities such as Fe, Al, and Cais 5 mass % or less) is preferable. It is most preferable to use a highpurity silicon powder having a Si content ratio of 99 mass % or more.There is no limitation regarding the shape of the silicon powder to beused, and not only a powder of spherical particles or particle of asimilar shape, but also a powder that is an aggregate of particleshaving irregular shape prepared by, for example, a roll milling or stampmilling can be used preferably.

[0028] When a yttrium oxide (Y₂O₃) powder and/or an aluminum oxide(Al₂O₃) powder is added to the material (composition) for forming aceramic material (typically, both of these oxide compound powders areadded), oxide powders having an average particle size (based onmicroscope measurement or a precipitation method) of about 0.01 μm ormore and less than 5 μm are suitable. However, those having an averageparticle size equal to or smaller than that of the metal silicon powderand the silicon nitride powder and/or the silicon carbide powder thatcoexist are preferable. In particular, it is preferable to use a yttriumoxide powder and/or an aluminum oxide powder having an average particlesize of 0.1 μm or more and less than 1 μm.

[0029] When the silicon nitride powder and/or the silicon carbide powderare mixed with the metal silicon powder, it is preferable to avoidoxidation of the metal silicon and impurities, although the presentinvention is not limited thereto. For example, it is preferable to mixthese raw material powders (further pulverize them, if necessary) in anonoxidative gas atmosphere such as nitrogen gas or argon gas.

[0030] When the oxide powder (yttrium oxide powder and/or aluminum oxidepowder) is not added, it is preferable to mix 5 parts or more and lessthan 60 parts (more preferably 10 parts or more and less than 60, evenmore preferably 10 parts or more and less than 50 parts) of the metalsilicon powder with respect to 100 parts of the nonoxide ceramic powder(the total amount of the silicon nitride powder and the silicon carbidepowder) in the mass ratio. When making a microporous ceramic materialhaving an average micropore size (based on a mercury penetration method)of about 0.5 μm or more and less than 2 μm (typically, 0.8 μm or moreand less than 1.5 μm), it is preferable to mix 25 parts or more and lessthan 45 parts (more preferably, 35 parts or more and less than 45, andmost preferably about 40 parts) of the metal silicon powder with respectto 100 parts of the nonoxide ceramic powder in the mass ratio.

[0031] Alternatively, when the oxide powder (yttrium oxide powder and/oraluminum oxide powder) is added, it is preferable to mix 10 parts ormore and less than 100 parts of the metal silicon powder with respect to100 parts of the nonoxide ceramic powder (the total amount of thesilicon nitride powder and the silicon carbide powder) in the massratio. In order to reduce the production cost, it is preferable to add50 parts or more and less than 100 parts of the metal silicon powderwith respect to 100 parts of the nonoxide ceramic powder. When only asilicon nitride powder is used as the nonoxide ceramic powder, it ispreferable to mix the silicon nitride powder and the metal siliconpowder such that 10 vol % or more and less than 50 vol % (morepreferably 15 vol % or more and less than 50 vol %, even more preferably20 vol % or more and less than 50 vol %) of the silicon nitrideconstituting the nonoxide microporous ceramic material obtained bynitriding and firing (i.e., reaction sintering) are derived from themetal silicon (nitride component). For example, 10 parts or more (whichcan be 9.5 parts or more) and less than 90 parts, more preferably 15parts or more and less than 90 parts, and even more preferably 20 partsor more and less than 90 parts of the metal silicon powder in the massratio are added with respect to 100 parts of the silicon nitride powder.

[0032] Regarding the oxide powder, it is preferable to add a yttriumoxide powder and/or an aluminum oxide powder in an amount correspondingto 2 mass % or more and less than 250 mass % of the content of the metalsilicon powder and not more than 20 mass % of the total amount of themetal silicon powder, the nonoxide ceramic powder and/or the oxidepowder. In order to reduce the production cost, it is preferable to adda yttrium oxide powder and/or an aluminum oxide powder in an amountcorresponding to 5 mass % or more and less than 100 mass % of thecontent of the metal silicon powder.

[0033] For the microporous ceramic material that is preferable for theabove-described applications (a filter, a catalyst carrier, a substrate(support) of a separation membrane or the like), the average microporesize or the peak value in the micropore size distribution is in therange approximately from 0.5 μm or more and less than 2.0 μm, morepreferably 0.6 μm or more and less than 1.6 μm, and most preferably 0.8μm or more and less than 1.5 μm, further 0.8 μm or more and less than1.2 μm, although the present invention is not limited thereto. In orderto maintain the mechanical strength, it is preferable that the porosity(based on a mercury penetration method) is smaller than 45 vol %, morepreferably 30 vol % or more and less than 40 vol %, and most preferably35 vol % or more and less than 40 vol %.

[0034] When making a microporous ceramic material having such an averagemicropore size or peak value of the micropore size distribution (basedon a mercury penetration method) and porosity in such preferable valueranges, it is preferable to mix a metal silicon powder in an amount inthe mass ratio of 20 parts or more and less than 95 parts (mostpreferably 30 parts or more and less than 95 parts) with respect to 100parts of the nonoxide ceramic powder. In this case, there is noparticular limitation regarding the amount of yttrium oxide powderand/or aluminum oxide powder to be added, as long as it is in theabove-described range. However, it is preferable that the total of theyttrium oxide powder and the aluminum oxide powder corresponds to anamount of 10 mass % or more and less than 100 mass % of the metalsilicon powder and not more than 20 mass % of the metal silicon powder,the nonoxide ceramic powder and the oxide powder. It is preferable toadd these two powders such that the molar ratio (Y₂O₃/Al₂O₃) of theyttrium oxide and the aluminum oxide is in the range from about 0.8 ormore and less than 1.2 (more preferably 0.9 or more and less than 1.1).

[0035] These raw material powders (metal silicon powder, nonoxideceramic powder and, if desired, an oxide powder) for preparing amaterial (composition) for forming a ceramic material can be mixed bycommonly used mixing means such as a ball mill, a mixer or the like.

[0036] In addition to the raw material powders, various additives can beadded to the material (composition) as appropriate. For example, varioussintering additives can be added for the purpose of suppressing graingrowth or stabilizing the microporous structure.

[0037] Furthermore, an appropriate binder can be added, depending on themolding method to be used. For example, a material (composition) thatcan be preferably used for extrusion molding can be prepared by mixingan appropriate binder and a dispersion medium (e.g., water, variousorganic solvents including a low alcohol such as ethanol, and a mixedsolution of water and an organic solvent) with a mixture of the rawmaterial powders and kneading the obtained mixture. As such a binder,polyvinyl alcohol, methyl celluloses, polyethylene glycols, propyleneglycol, glycerin or the like can be used. As the mixing ratio of thebinder, about 5 parts or more and less than 30 parts with respect to 100parts of the mixture of the raw material powders in the mass ratio isappropriate, but the present invention is not limited thereto. As thekneading means, a kneader or various mixers (ribbon mixer, Henschelmixer or the like) can be used.

[0038] There is no particular limitation regarding a method for moldingthe obtained material to a desired shape, and conventionally commonlyused various ceramic molding methods can be used. For example, extrusionmolding, pressing, or casting can be used. Pressing (uniaxial pressing,hydrostatic pressing or the like) utilizing a floating die or a pressingmachine is preferable. Either hot pressing or cold pressing can be used.

[0039] There is no limitation regarding the molding pressure, whichdepends on the filling ratio of the material, but preferably a moldingpressure of about 30 MPa or more and less than 200 MPa (e.g., 100 MPa ormore and less than 150 MPa) can be used for pressing. By this pressing,a microporous ceramic material suitable for the above-listedapplications that has a relatively narrow micropore size distributioncan be made. When making a microporous ceramic material having anaverage micropore size (based on a mercury penetration method) of about0.5 μm or mor and less than 2 μm (typically, 0.8 μm or more and lessthan 1.5 μm, for example, 1 μm or more and less than 1.2 μm), a pressureof about 50 MPa or more and less than 200 MPa is preferable.Particularly preferably, pressure molding is performed at a pressure ofabout 100 MPa or more and less than 200 MPa. A high pressure such as 250MPa or more is not preferable because it leads to a significantreduction of the porosity (25% or less).

[0040] In the method disclosed herein, a molded product is fired in anatmosphere that allows nitriding (atmosphere based on nitrogen gas witha nitrogen partial pressure of 50 kPa or more, ammonia gas and the like,and substantially not including oxygen) at a typical gas pressure of 80kPa or more and less than 120 kPa (about 0.8 atm or more and less than1.2 atm) in a temperature range that allows nitriding (preferably 1200°C. or more and less than 1500° C.) for 2 hours or more and less than 12hours. When the firing temperature is higher than that, or the firingtime is longer than that, needle-like (fiber-like) β-silicon nitride isproduced and precipitated in a large amount in the sintered product, sothat the micropore size is smaller than a desired size or the microporesize distribution tends to be broad, which is not preferable.

[0041] For example, heating is performed in a nitrogen atmosphere at atemperature from room temperature to a middle temperature range (700° C.or more and less than 900° C.) at a temperature increase rate of 2°C./min or more and less than 10° C./min (preferably 5° C./min or moreand less than 7° C./min). Thereafter, heating is performed in anatmosphere that allows nitriding (typically at a temperature of 1200° C.or more) at a temperature increase rate of 1° C./min or more and lessthan 5° C./min. Thereafter, it is preferable to store the sinteredproduct in a temperature range that allows nitriding for about 2 hoursor more and less than 10 hours. During this storage, it is not necessaryto keep the temperature constant and the temperature can be varied asappropriate. For example, the sintered product may be stored at 1300° C.for two hours and then the temperature may be increased to 1500° C. overone hour, and then the sintered product may be stored at thattemperature for another one hour. By performing reaction sintering und rthe above conditions, α silicon nitride can be produced from the metalsilicon particles efficiently.

[0042] In the method disclosed herein, a crystalline silicon nitride isprimarily produced by the reaction sintering, and a nonoxide microporousceramic material suitable for the above applications can be made thathas a peak value of the micropore size distribution or an averagemicropore size in the range approximately from 0.5 μm or more and lessthan 2 μm (preferably 0.6 μm or more and less than 1.6 μm, and morepreferably 0.8 μm or more and less than 1.5 μm, and most preferably 0.8μm or more and less than 1.2 μm), and typically a porosity of 45 vol %or less (more preferably 30 vol % or more and less than 40 vol %, andmost preferably 35 vol % or more and less than 40 vol %).

[0043] According to the present invention, a microporous ceramicmaterial used as a filter, a catalyst carrier, a substrate of a ceramicmembrane for gas or liquid separation or the like can be provided at acomparatively low cost by utilizing a metal silicon powder, which isless expensive than a silicon carbide powder or a silicon nitridepowder. The present invention also provides a method for making amicroporous ceramic substrate for a ceramic membrane for gas separation.Furthermore, a ceramic membrane module for gas separation (gasseparating module) using this ceramic material as a support can beprovided.

[0044] The present invention will now be described more specifically byway of several examples, but the present invention is not limitedthereto.

EXAMPLE 1 Production of Microporous Ceramic Materials

[0045] Materials for forming a ceramic material having mixing ratiosshown in Tables 1 and 2 (i.e., Samples Nos. 1 to 6 and Comparativesamples Nos. 1 to 6 for which a SiC powder was used as the nonoxideceramic powder, and Samples Nos. 7 and 8 and Comparative samples No. 7for which a Si₃N₄ powder was used as the nonoxide ceramic powder) wereprepared, using a high purity metal silicon powder (Si purity: 96 mass %or more, an average particle size: about 12 μm), a silicon carbidepowder having an average particle size of about 40 μm (SiC purity: 95mass % or more, free carbon content: 1 mass % or less, hereinafterreferred to as “40μ-SiC powder”), a silicon carbide powder having anaverage particle size of about 5 μm (SiC purity: 95 mass % or more, freecarbon content: 1 mass % or less, hereinafter referred to as “5μ-SiCpowder”) or a silicon nitride powder having an average particle size ofabout 4 μm (Si₃N₄ purity: 95 mass % or more, hereinafter referred to as“4μ-Si₃N₄ powder”). TABLE 1 mixing ratio (g) 40μ-SiC 5μ-SiC metalsilicon (particle size (particle size (particle size Sample No. about 40μm) about 5 μm) about 12 μm) Sample No.1 50 —  5 Sample No.2 50 —  10Sample No.3 50 —  20 Sample No.4 — 50  5 Sample No.5 — 50  10 SampleNo.6 — 50  20 Com. Sample No.1 50 —  45 Com. Sample No.2 50 — 110 Com.Sample No.3 — 50  30 Com. Sample No.4 — 50  45 Com. Sample No.5 — 50 110Com. Sample No.6 — — 100

[0046] TABLE 2 mixing ratio (g) 4μ-Si₃N₄ (particle size metal siliconSample No. about 4 μm) (particle size about 12 μm) Sample No.7 50 4.8Sample No.8 50 9.4 Com. Sample No.7 50 41.2 

[0047] More specifically, the 40μ-SiC powder, the 5μ-SiC powder or th4μ-Si₃N₄ powder were mixed with the metal silicon powder in the amountsshown in Tables 1 and 2, and water was added in an appropriate amount,and a mixture was blended in a ball mill for 30 minutes. Comparativesample No. 6 in Table 1 was prepared by placing 100 g of a metal siliconpowder and an appropriate amount of water in a ball mill without addingeither a SiC powder or a Si₃N₄ powder, and performing the sameprocedure.

[0048] After drying, the obtained mixed material was filled in a pressdie having a diameter of 29 mm (sample weight: about 3 g).

[0049] Then, uniaxial pressing was performed at a pressure of about 100MPa (1000 kg/cm²). Each of the obtained molded products was placed in anelectric furnace that was filled with a nitrogen atmosphere, and wasfired in the following temperature schedule. That is, the temperaturewas increased from room temperature to 800° C. over two hours(temperature increase rate: 5 to 7° C./min), then the temperature wasincreased to 1200° C. over two hours (temperature increase rate: 2 to 4°C./min). Thereafter, the temperature was increased to 1375° C. over onehour (temperature increase rate: 2 to 4° C./min) and was maintained at1375° C. for two hours. Thereafter, the temperature was increased to1500° C. over one hour (temperature increase rate: 1 to 3° C./min) andwas maintained at 1500° C. for one hour. Thereafter, the temperature wasdecreased gradually to room temperature. With this series of processes,disk-like microporous ceramic materials having a diameter of 29 mm and aheight of 2 mm corresponding to the die were obtained from each material(Samples Nos. 1 to 8, Comparative samples Nos. 1 to 7). In the followingdescription, these microporous ceramic materials are denoted by the samesample number as that of the material used.

[0050] The micropore diameter and the porosity of each ceramic materialobtained by the above procedure were investigated. More specifically,using a commercially available mercury porosimeter (Autopore III(product name) manufactured by Micromeritics Instrument Corporation),the micropore size distribution and the peak value thereof and theporosity were obtained based on a mercury penetration method.

[0051] Furthermore, for Samples Nos. 1 to 6 and Comparative samples Nos.1 to 6, the volume ratio of the silicon carbide (SiC) and the siliconnitride (Si₃N₄) was calculated from the mixing ratio of the raw materialpowders (the amount of the raw material powders used). For Samples Nos.7 and 8 and Comparative sample No.7, the volume ratio of the siliconnitride (raw material component, that is, derived from the Si₃N₄ powder)and the silicon nitride produced by nitriding of Si (nitride component,that is, derived from the metal silicon) was calculated from the mixingratio of the raw materials (the amount of the raw materials used).Tables 3 and 4 show the results. TABLE 3 ceramic volume ratio (vol %)micropore material silicon silicon size porosity sample No. carbidenitride peak (μm) (vol %) Sample No.1 90 10 5.86 44 Sample No.2 82 183.08 36 Sample No.3 70 30 1.07 37 Sample No.4 90 10 1.76 52 Sample No.582 18 1.29 48 Sample No.6 70 30 1.17 45 Com. Sample No.1 51 49 0.17 32Com. Sample No.2 30 70 0.12 31 Com. Sample No.3 61 39 0.24 40 Com.Sample No.4 51 49 0.13 37 Com. Sample No.5 30 70 0.16 34 Com. SampleNo.6  0 100  0.14 38

[0052] TABLE 4 volume ratio (vol %) ceramic silicon nitride siliconnitride micropore material (derived from raw (produced by size peakporosity sample No. material) nitriding) (μm) (vol %) Sample No.7 90 100.7  38 Sample No.8 82 18 0.5  37 Com. Sample 51 49 0.15 36 No.7

[0053] As shown in Table 3, for all the microporous ceramic materials ofSamples Nos. 1 to 6, the peak value of the micropore size distributionwas not less than 1 μm, and the micropore size distribution was narrowwith the peak value as the center. The porosity was about 35 vol % ormore and less than 50 vol %. Therefore, these ceramic materials can beused preferably as a gas or liquid filter or a catalyst carrier throughwhich gas or air can permeate. In particular, the ceramic materials ofSamples Nos. 3 and 6 have a peak value of the micropore sizedistribution in the range from 0.8 μm or more and less than 1.2 μm, andcan be used particularly preferably as a substrate of a ceramic membraneused for gas separation. On the other hand, the formed micropore size ofthe ceramic materials of Comparative samples Nos. 1 to 6 was too small,so that it is believed that they are not suitable for the aboveapplications.

[0054] Furthermore, as shown in Table 4, for the microporous ceramicmaterials of Samples Nos.7 and 8, the peak value of the micropore sizedistribution was not less than 0.5 μm, and the micropore sizedistribution was narrow with the peak value as the center. The porositywas about 35 vol % or more and less than 40 vol %. Therefore, theseceramic materials as well as the ceramic materials of Samples Nos. 1 to6 can be used preferably as a gas or liquid filter or a catalyst carrierthrough which gas or air can permeate. In particular, the ceramicmaterial of Sample No. 7 has a peak value of the micropore sizedistribution of 0.7 μm and can be used particularly preferably as asubstrate of a ceramic membrane for gas separation under hightemperature conditions. On the other hand, the formed micropore size ofthe ceramic material of Comparative sample No. 7 was too small, so thatit is believed that it is not suitable for the above applications.

[0055] The surface of the ceramic material of Sample No. 6 (FIG. 1) andthe surface of the ceramic material of Comparative sample No. 1 (FIG. 3)were observed with an electron microscope (SEM). The results confirmedthat a large amount of needle-like β-silicon nitride was present on thesurface of the ceramic material of Comparative sample No. 1, and theceramic material of Comparative sample No. 1 had a comparatively densestructure. On the other hand, it was confirmed that substantially noneedle-like β silicon nitride was present on the surface of the ceramicmaterial of Sample No. 6, and the ceramic material of Sample No. 6 had amicropore-rich structure.

EXAMPLE 2 Production of Microporous Ceramic Materials

[0056] Next, microporous ceramic materials were made using powdermaterials (compositions) containing a silicon carbide powder as thenonoxide ceramic powder, and a yttrium oxide powder and an aluminumoxide powder as the oxide powder. More specifically, materials forforming a ceramic material having mixing ratios shown in Table 5 (i.e.,Samples Nos. 9 to 12) were prepared, using the same high purity metalsilicon powder as that used in Example 1 (Si purity: 96 mass % or more,an average particle size: about 12 μm), the 5μ-SiC powder (SiC purity:95 mass % or more, free carbon content: 1 mass % or less), a yttriumoxide powder having an average particle size of about 1 μm and analuminum oxide powder having an average particle size of about 0.3 μm.TABLE 5 mixing ratio (g) silicon carbide metal silicon aluminum oxideyttrium oxide (particle size (particle size (particle size (particlesize Sample No. about 5 μm) about 12 μm) about 0.3 μm) about 1 μm)Sample No.9 50  5 0.3 0.3 Sample No.10 50 10 0.6 0.6 Sample No.11 50 201.2 1.2 Sample No.12 50 45 2.7 2.7 Com. Sample No.3 50 30 — — Com.Sample No.4 50 45 — — Com. Sample No.5 50 110  — — Com. Sample No.6 —100  — —

[0057] More specifically, the 5μ-SiC powder and the metal silicon powderwere mixed in the amounts shown in Table 5, and further the yttriumoxide powder and the aluminum oxide powder were added such that the massratio of each powder was 6 mass % with respect to the amount of thesilicon powder, and water was added in an appropriate amount. Then, amixture was blended in a ball mill for 30 minutes. Thus, the materialsof Samples Nos. 9 to 12 were prepared. Comparative samples Nos. 3 to 6in Table 5 are the same materials as shown in Table 1.

[0058] After drying, in the same manner as in Example 1, each materialwas filled in a press die having a diameter of 29 mm (sample weight:about 3 g). Then, uniaxial pressing was performed at a pressure of about100 MPa. Each of the obtained molded products was placed in an electricfurnace that was filled with a nitrogen atmosphere, and was fired in thesame temperature schedule as described in Example 1. With this series ofprocesses, disk-like microporous ceramic materials having a diameter of29 mm and a height of 2 mm corresponding to the die were obtained fromeach material (Samples Nos. 9 to 12). In the following description,these microporous ceramic materials are denoted by the same samplenumber as that of the material used.

[0059] The micropore diameter and the porosity of each ceramic materialobtained by the above procedure were investigated in the same manner asdescribed in Example 1. Furthermore, the volume ratio of the siliconcarbide (SiC) and the silicon nitride (Si₃N₄) of each ceramic materialwas calculated from the mixing ratio of the raw material powders (theamount of the raw material powders used). Table 6 shows the results.TABLE 6 volume ratio (vol %) ceramic material silicon silicon averagemicropore porosity sample No. carbide nitride size (μm) (vol %) SampleNo.9 90 10 1.62 44 Sample No.10 82 18 1.36 42 Sample No.11 70 30 0.90 36Sample No.12 51 49 1.15 36 Com. Sample No.3 61 39 0.24 40 Com. SampleNo.4 51 49 0.13 37 Com. Sample No.5 30 70 0.16 34 Com. Sample No.6 —100  0.14 38

[0060] As shown in Table 6, for the microporous ceramic materials ofSamples Nos. 9 to 12, the average micropore size was about 0.9 μm to 1.6μm, and the micropore size distribution was narrow with that averagemicropore size substantially as the center. The porosity was about 35vol % to 45 vol %. Therefore, these ceramic materials can be usedpreferably as a gas or liquid filter or a catalyst carrier through whichgas or air can permeate. In particular, the ceramic materials of SamplesNos. 11 and 12 have an average micropore size in the range from 0.8 μmto 1.2 μm, and can be used particularly preferably as a substrate of aceramic membrane for gas separation.

EXAMPLE 3 Production of Microporous Ceramic Materials

[0061] Next, microporous ceramic materials were made using powdermaterials (compositions) containing a silicon nitride powder as thenonoxide ceramic powder, and a yttrium oxide powder and an aluminumoxide powder as the oxide powder. More specifically, materials forforming a ceramic material having mixing ratios shown in Table 7 (i.e.,Samples Nos. 13 to 18) were prepared, using the same high purity metalsilicon powder as that used in Example 1 (Si purity: 96 mass % or more,an av rage particle size: about 12 μm), the 4μ-Si₃N₄ powder (Si₃N₄purity: 95 mass % or more), a yttrium oxide powder having an averageparticle size of about 1 μm and an aluminum oxide powder having anaverage particle size of about 0.3 μm. TABLE 7 mixing ratio (%) aluminumoxide metal (particle 4μ-Si₃N₄ silicon size yttrium oxide (particle size(particle size about (particle size Sample No. about 4 μm) about 12 μm)0.3 μm) about 1 μm) Sample No.13 50 4.8 3.2 7.1 Sample No.14 50 4.8 0.30.3 Sample No.15 50 9.4 3.5 7.8 Sample No.16 50 9.4 0.6 0.6 Sample No.1750 18.4  4.1 9.0 Sample No.18 50 41.2  5.7 12.5 

[0062] More specifically, the 4μ-Si₃N₄ powder and the metal siliconpowder were mixed in the amounts shown in Table 7, and further theyttrium oxide powder and the aluminum oxide powder in the amounts shownin Table 7 with respect to the amount of the silicon powder were added,and water was added in an appropriate amount. Then, a mixture wasblended in a ball mill for 30 minutes. Thus, the materials of SamplesNos. 13 to 18 were prepared. Among these, for Samples Nos. 14 and 16,the yttrium oxide powder and the aluminum oxide powder were added suchthat the molar ratio (Y₂O₃/Al₂O₃) of the yttrium oxide and the aluminumoxide was about ½, that is, 0.5. For the other samples (Nos. 13, 15, 17and 18), these powders were added such that the molar ratio (Y₂O₃/Al₂O₃)was about 1/1, that is, 1.

[0063] After drying, in the same manner as in Examples 1 and 2, eachmaterial was filled in a press die having a diameter of 29 mm (sampleweight: about 3 g). In this example, a plurality of molded products wasprepared from each material. Then, uniaxial pressing was performed atabout 50 MPa (500 kg/cm²) for some of the plurality of molded products,at about 100 MPa (1000 kg/cm²) for some of them, and at about 150 MPa(1500 kg/cm²) for the rest of them. Each of the obtained molded productswas placed in an electric furnace that was filled with a nitrogenatmosphere, and was fired in the same temperature schedule as describedin Example 1. With this series of processes, disk-like microporousceramic materials having a diameter of 29 mm and a height of 2 mmcorresponding to the die were obtained from each material (Samples Nos.13 to 18). In the following description, these microporous ceramicmaterials are denoted by the same sample number and the same moldingpressure level as those of the materials used. For example, No. 13-50 inTable 8 shows a microporous ceramic material obtained by using thematerial of Sample No. 13 and performing uniaxial pressing at about 50MPa. TABLE 8 volume ratio (vol %) silicon silicon nitride nitrideceramic (derived (produced molding average material from by pressuremicropore porosity sample No. raw material) nitriding) (MPa) size (μm)(vol %) No. 13-50 90 10 50 0.80 36 No. 13-100 90 10 100 0.72 34 No.13-150 90 10 150 0.70 34 No. 14-50 90 10 50 0.77 42 No. 14-100 90 10 1000.67 40 No. 14-150 90 10 150 0.64 40 No. 15-50 82 18 50 0.80 38 No.15-100 82 18 100 0.71 34 No. 15-150 82 18 150 0.68 33 No. 16-50 82 18 500.81 41 No. 16-100 82 18 100 0.71 38 No. 16-150 82 18 150 0.65 38 No.17-50 70 30 50 0.83 36 No. 17-100 70 30 100 0.75 33 No. 17-150 70 30 1500.75 33 No. 18-50 51 49 50 0.81 36 No. 18-100 51 49 100 0.71 34 No.18-150 51 49 150 0.66 32

[0064] The micropore diameter and the porosity and the volume ratio ofthe silicon nitride portion derived from the raw material component andthe silicon nitride portion newly produced by nitriding (reactionsintering) of each of the obtained ceramic materials were investigatedin the same manner as described above. Table 8 shows the results. Asshown in Table 8, for all the microporous ceramic materials made in thisexample, the average micropore size was about 0.6 μm or more, and themicropore size distribution was narrow with that average micropore sizesubstantially as the center. The porosity was about 30 vol % to 45 vol%. Therefore, these ceramic materials can be used preferably as a gas orliquid filter or a catalyst carrier through which gas or air canpermeate. In particular, the ceramic materials of Samples Nos. 13-50,15-50, 16-50, 17-50 and 18-50 have an average micropore size of 0.8 μmor more, and can be used particularly preferably as a substrate of aceramic membrane used for gas separation.

[0065] Furthermore, as a result of using the same materials and varyingthe pressure level (50 MPa, 100 MP and 150 MPa) of the uniaxialpressing, it was confirmed that a molding pressure of about 50 MPa ispreferable to make a ceramic material having a comparatively largemicropore size (Table 8). Furthermore, from comparison between the caseusing the material of Sample No. 13 and the case using the material ofSample No. 14 and between the case using the material of Sample No. 15and the case using the material of Sample No. 16, it is preferable toadd the yttrium oxide powder and the aluminum oxide powder such that themolar ratio (Y₂O₃/Al₂O₃) was about 1 in order to realize the porosity ofabout 35 vol % to 40 vol %.

[0066] The surface of the ceramic material of Sample No. 18-50 wasobserved with an electron microscope (SEM). As shown in FIG. 2,substantially no needle-like β silicon nitride was present on thesurface of the ceramic material of Sample No. 18-50, and the ceramicmaterial of Sample No. 18-50 had a structure that was rich in microporeshaving a size of 0.8 μm to 1.2 μm, which is suitable as a substrate of agas separation membrane.

[0067] Furthermor, using Samples 13 and 17 as the material for forming amicroporous ceramic material, microporous ceramic materials having ashape of a test piece for three-point bending strength (a straight beamshape having a width of 4 mm, a thickness of 3 mm, and a total length of40 mm) according to JIS were made, and the three-point bending strengthwas measured according to JIS (R1601).

[0068] More specifically, each powder material (Sample No. 13 or 17) wasfilled in a press die corresponding to the shape of the test piece. Inthis example, a plurality of molded products was prepared from eachmaterial. Then, uniaxial pressing was performed at a pressure of about20 MPa (200 kg/cm²) for some of the plurality of molded products, atabout 50 MPa (500 kg/cm²) for some of them, at about 100 MPa (1000kg/cm²) for some of them, and at about 150 MPa (1500 kg/cm²) for therest of them. Each of the obtained molded products was placed in anelectric furnace that was filled with a nitrogen atmosphere, and wasfired in the same temperature increase schedule as described inExample 1. Thus, a large number of microporous ceramic materials havinga shape of a three-point bending strength test piece corresponding tothe mold were obtained. In the following description, these microporousceramic materials are denoted by the same sample number and the samemolding pressure level as those of the materials used. For example, No.13-20 in Table 9 shows a microporous ceramic material (test piece formeasuring a three-point bending strength) obtained by using the materialof Sample No. 13 and performing uniaxial pressing at about 20 MPa.

[0069] Thus, each of the obtained test piece was subjected to athree-point bending strength test according to JIS under the roomtemperature (25° C.) condition and a high temperature (800° C.)condition. Table 9 shows the results. TABLE 9 volume ratio (vol %)three-point three-point silicon nitride silicon nitride molding bendingbending test piece (derived from (produced by pressure strength (MPa)strength (MPa) sample No. raw material) nitriding) (MPa) (25° C.) (800°C.) No. 13-20 90 10 20 14 13 No. 13-50 90 10 50 17 16 No. 13-100 90 10100 27 28 No. 13-150 90 10 150 33 34 No. 17-20 70 30 20 31 31 No. 17-5070 30 50 44 44 No. 17-100 70 30 100 55 59 No. 17-150 70 30 150 81 84

[0070] As shown in Table 9, in either case of the materials, thematerial produced at a higher molding pressure has a higher bendingstrength, regardless of the temperature (Sample No.13-20<13-50<13-100<13-150, and Sample No. 17-20<17-50<17-100<17-150). Inparticular, the microporous ceramic material (bending strength testpiece) made of Sample No. 17 (see Table 7 with respect to thecomposition) achieved a bending strength of 40 MPa or more at a moldingpressure of 50 MPa, a bending strength of 50 MPa or more at a moldingpressure of 100 MPa, and a bending strength of 80 MPa or more at amolding pressure of 150 MPa.

[0071] Specific examples of the present invention have been describedabove, but they are only illustrative and are not limiting to the scopeof these claims. All changes and modifications from the specificexamples illustrated above are intended to be embraced in the techniquesdisclosed in the appended claims. The technical elements described inthe specification or the drawings, can exhibit technical usefulness,either alone or in combination, and combinations are not limited tothose described in the claims as filed. The techniques illustrated inthe specification or the drawings can achieve a plurality of purposes atthe same time, and achieving only one of them has technical usefulness.

What is claimed is:
 1. A composition for forming a microporous ceramicmaterial comprising: a metal silicon powder, and at least one nonoxideceramic powder selected from the group consisting of a silicon nitridepowder and a silicon carbide powder, wherein a mixing ratio of the metalsilicon powder and the nonoxide ceramic powder is 5 parts or more andless than 60 parts of the metal silicon powder with respect to 100 partsof the nonoxide ceramic powder in a mass ratio.
 2. The compositionaccording to claim 1, wherein a mixing ratio of the metal silicon powderand the nonoxide ceramic powder is 25 parts or more and less than 45parts of the metal silicon powder with respect to 100 parts of thenonoxide ceramic powder in a mass ratio.
 3. The composition according toclaim 1, wherein an average particle size of the metal silicon powderand the nonoxide ceramic powder is in a range from 1 μm or more and lessthan 50 μm.
 4. The composition according to claim 1, comprising adispersion medium that disperses the metal silicon powder and thenonoxide ceramic powder.
 5. A composition for forming a microporousceramic material comprising: a metal silicon powder, at least onenonoxide ceramic powder selected from the group consisting of a siliconnitride powder and a silicon carbide powder, and at least one oxidepowder selected from the group consisting of a yttrium oxide powder andan aluminum oxide powder, wherein a mixing ratio of the metal siliconpowder and the nonoxide ceramic powder is 10 parts or more and less than100 parts of the metal silicon powder with respect to 100 parts of thenonoxide ceramic powder in a mass ratio, and the content of the oxidepowder is an amount corresponding to 2 mass % or more and less than 250mass % of the content of th metal silicon powder and not more than 20mass % of the total amount of the metal silicon powder, the nonoxideceramic powder and the oxide powder.
 6. The composition according toclaim 5, wherein a mixing ratio of the metal silicon powder and thenonoxide ceramic powder is 20 parts or more and less than 90 parts ofthe metal silicon powder with respect to 100 parts of the nonoxideceramic powder in a mass ratio.
 7. The composition according to claim 5,wherein an average particle size of each of the metal silicon powder andthe nonoxide ceramic powder is in a range from 1 μm or more and lessthan 50 μm.
 8. The composition according to claim 5, wherein an averageparticle size of the oxide powder is in a range from 0.1 μm or more andless than 1 μm.
 9. The composition according to claim 5, comprising adispersion medium that disperses the metal silicon powder, the nonoxideceramic powder and the oxide powder.
 10. A method for making amicroporous ceramic material comprising: preparing a compositioncomprising a metal silicon powder, and at least one nonoxide ceramicpowder selected from the group consisting of a silicon nitride powderand a silicon carbide powder, wherein a mixing ratio of the metalsilicon powder and the nonoxide ceramic powder is 5 parts or more andless than 60 parts of the metal silicon powder with respect to 100 partsof the nonoxide ceramic powder in a mass ratio, molding the compositioninto a molded product having a predetermined shape, and subjecting themolded product to reaction sintering in an atmosphere that allowsnitriding.
 11. The method according to claim 10, wherein a mixing ratioof the metal silicon powder and the nonoxide ceramic powder in thecomposition is 25 parts or more and less than 45 parts of the metalsilicon powder with respect to 100 parts of the nonoxide ceramic powderin a mass ratio.
 12. The method according to claim 10, wherein anaverage particle size of each of the metal silicon powder and thenonoxide ceramic powder contained in the composition is in a range from1 μm or more and less than 50 μm.
 13. The method according to claim 10,wherein the composition is molded under pressure at a molding pressureset in a range from 30 MPa or more and less than 200 MPa in the moldingprocess.
 14. A method for making a microporous ceramic materialcomprising: preparing a composition comprising a metal silicon powder,at least one nonoxide ceramic powder selected from the group consistingof a silicon nitride powder and a silicon carbide powder, and at leastone oxide powder selected from the group consisting of a yttrium oxidepowder and an aluminum oxide powder, wherein a mixing ratio of the metalsilicon powder and the nonoxide ceramic powder is 10 parts or more andless than 100 parts of the metal silicon powder with respect to 100parts of the nonoxide ceramic powder in a mass ratio, and the content ofthe oxide powder is an amount corresponding to 2 mass % or more and lessthan 250 mass % of the content of the metal silicon powder and not morethan 20 mass % of the total amount of the metal silicon powder, thenonoxide ceramic powder and the oxide powder, molding the compositioninto a molded product having a predetermined shape, and subjecting themolded product to reaction sintering in an atmosphere that allowsnitriding.
 15. The method according to claim 14, wherein a mixing ratioof the metal silicon powder and the nonoxide ceramic powder in thecomposition is 20 parts or more and less than 90 parts of the metalsilicon powder with respect to 100 parts of the nonoxide ceramic powderin a mass ratio.
 16. The method according to claim 14, wherein anaverage particle size of each of the metal silicon powder and thenonoxide ceramic powder contained in the composition is in a range from1 μm or more and less than 50 μm.
 17. The method according to claim 14,wherein an average particle size of the oxide ceramic powder containedin the composition is in a range from 0.1 μm or more and less than 1 μm.18. The method according to claim 14, wherein the composition is moldedunder pressure at a molding pressure set in a range from 30 MPa or moreand less than 200 MPa in the molding process.
 19. A microporous ceramicmaterial made by the method according to claim
 10. 20. A microporousceramic material made by the method according to claim 14.